Warders in a French jail where prisoners are allowed to grow their own produce have been left red-faced after the discovery of cannabis plants among the flowers and vegetables.

The plants, which had reached a height of around 80cm (two and a half feet) before being spotted at the weekend, were found in vegetable patches located in the exercise courtyard of the Saint-Martin-de-Re prison in the west of France, Christophe Beaulieu, a regional official of the prison officers' union, told AFP.

"When you don't know what you are looking for it is easy to confuse them with other plants," Beaulieu said.